(i) Field of the Invention
This invention relates to a galvanizing alloy and process and, more particularly, relates to a galvanizing alloy and an immersion galvanizing process for providing a decorative coating to non-reactive and to moderately reactive or mixed reactive steels.
(ii) Description of the Related Art
It is necessary, in the manufacture of low-alloy high-strength steels by the continuous casting process, to add elements that ‘kill’ or deoxidize the steel i.e. prevent gaseous products which produce porosity. Silicon is commonly employed for this purpose. These steels, as a result, generally contain between 0.01% to 0.3%, by weight, silicon but may include up to or more than about 0.5 wt % silicon and are known as ‘reactive steels’ or silicon steels.
Phosphorus in the steel also affects reactivity having an accepted measure of reactivity that is approximately 2.5 times that of silicon. Thus, the silicon content plus 2.5 times the phosphorus content is known as the effective silicon content of the steel.
Steels with silicon levels between 0.05 to 0.15 (i.e. around the first “Sandelin Peak” area), may also develop a ‘mixed’ reactivity coating. This coating is characterized by a combination of reactive and non-reactive areas on the same steel which is believed to be due to differences in localized silicon levels on the surface of the steel.
Silicon steels that have high reactivity pose problems to the galvanizing process, producing thick, brittle and uneven coatings, poor adherence and/or a dull or marbled appearance. These coatings are known as ‘reactive’ coatings. The high reactivity of the silicon steels also causes excessive zinc consumption and excessive dross formation.
Silicon released from the steel during galvanizing is insoluble in the intermetallic layer known as the zeta layer. This creates an instability in the zeta layer and produces thick, porous intermetallic layers. The microstructure is characterized by a very thin and uneven delta layer overlaid by a very thick and porous zeta layer. The porous intermetallic layer allows liquid bath metal to react near the steel interface during the entire immersion period. The result is a linear growth mode with immersion time that allows the formation of excessively thick coatings. These coatings are generally very rough, undesirably thick, brittle and dull in appearance.
It is known to control steel reactivity by adding alloying elements to the zinc galvanizing bath. One such addition is nickel in a process known as the Technigalva™ (or Nickel-Zinc) process. A nickel content of about 0.05 to 0.10% by weight in the zinc bath effectively controls reactive steels having up to about 0.25% by weight silicon content. For steels having silicon levels above approximately 0.25 wt %, this nickel-zinc process is not effective and thus it is only a partial solution to the reactive steel galvanizing problem. Low reactivity (normal) steels, when galvanized by the nickel-zinc process, pose the same difficulty as seen in low temperature galvanizing in that coating thickness may be unacceptably thin. With this process, it is thus preferred that the galvanizer know the reactivity of the steel beforehand and adjust galvanizing conditions accordingly, both of which are difficult to accomplish in practice. Under some conditions, this process also produces dross that tends to float in the bath and be drawn out on the workpiece, producing unacceptable coatings.
Another alloy used to control reactivity is that disclosed in French Patent No. 2,366,376, granted Oct. 27, 1980, for galvanizing reactive steels, known as the Polygalva™ process. The alloy comprises zinc of commercial purity containing by weight 0.1 to 1.5% lead, 0.01 to 0.05% aluminum, 0.03 to 2.0% tin, and 0.001 to 2.0% magnesium.
U.S. Pat. No. 4,439,397, granted Mar. 27, 1984, discusses the accelerated rate at which the magnesium and aluminum are consumed or lost in this Polygalva™ process for galvanizing steel. Procedures are presented to overcome the inherent difficulty in replenishing deficient aluminum or magnesium in the zinc alloy galvanizing bath. The process has serious limitations in that the steel has to be meticulously degreased, pickled, pre-fluxed and oven-dried to obtain good quality product free of bare spots. Thus, in most cases, new high-quality installations are usually required.
U.S. Pat. No. 4,168,972, issued Sep. 25, 1979, and U.S. Pat. No. 4,238,532, issued Dec. 9, 1980, also disclose alloys for galvanizing reactive steels. The alloys presented include variations of the Polygalva™ alloy components of lead, aluminum, magnesium and tin in zinc.
It is known in the prior art that aluminum included in the galvanizing bath reduces the reactivity of the high silicon steels. A process known as the Supergalva™ process includes an alloy of zinc containing 5 wt % aluminum. The process requires a special flux and double dipping not generally accepted by commercial galvanizers.
Co-Pending U.S. patent application Ser. No. 08/667,830 filed Jun. 20, 1996, describes a new alloy and process for controlling reactivity in steels with silicon content up to 1 wt %. The alloy comprises zinc of commercial purity containing, by weight, one or both of vanadium in the amounts of at least 0.02% to 0.04% and titanium in the amounts of at least 0.02% to 0.05%.
Co-pending U.S. patent application Ser. No. 09/445,144 filed Feb. 22, 2000, describes a new alloy and process for controlling reactivity in steels in silicon contents up to 1 wt % in which the alloy comprises, by weight, aluminum in the amount of at least 0.001%, tin in the amount of at least 0.5% to a maximum of 2%, preferably at least 0.8%, and one of an element selected from the group consisting of vanadium in the amount of at least 0.02%, preferably 0.05% to 0.12%, titanium in the amount of at least 0.03%, preferably 0.06% to 0.10%, and both vanadium and titanium together in the amount of at least 0.02% vanadium and at least 0.01% titanium for a total of at least 0.03%, preferably 0.05 wt % to 0.15%, of vanadium and titanium, the balance zinc.
PCT Application No. PCT/BE98/00075 discloses a zinc alloy for galvanizing reactive steel comprising 1 to 5 wt % tin+bismuth, 0 to saturation of lead, 0.025 to 0.2 wt % of at least one of nickel, chromium or manganese, 0 to 0.03 wt % of at least one of aluminum, calcium and magnesium, the balance zinc. PCT Application No. PCT/EP97/00864 discloses a zinc alloy for galvanizing reactive steel comprising either 3 to 15 wt % tin or 1 to 5 wt % tin and 0.01 to 0.1 wt % nickel, lead up to saturation, and 0.06 wt % of at least one of aluminum, calcium and magnesium, the balance zinc.
The above prior art is directed at highly reactive steels. The tin contents of these alloy baths is high and, in that tin and bismuth are relatively expensive metals, it is economically desirable to provide an alloy for decorative galvanized coatings for non-reactive and mixed and moderately reactive steels having reduced amounts of tin and bismuth.
It is known in the prior art to produce coloured zinc coatings on metal and non-metallic surfaces. U.S. Pat. No. 3,530,013, issued Sep. 22, 1970, discloses a galvanizing zinc coating having minor amounts of an oxygen-avid element such as titanium, manganese or vanadium which is oxidized under controlled time and temperature conditions for provision of a surface film of an oxide of the oxygen-avid element having light interference colour characteristics.